The present invention relates to a method for dispensing viscous materials, and more particularly, to an automated method for rapid precision dispensing of minute amounts of adhesives, solder paste, and other flowable materials a constant height above the surface of a printed circuit board or other workpiece.
In the manufacture of electronic systems and equipment, it is frequently necessary to apply small amounts of viscous or flowable materials onto circuit boards and other substrates. Such materials may include adhesives, solder paste, epoxy, cyanoacrylates, RTV, silicones, solder mask, surface mount adhesive flux, grease, oil, encapsulants, potting compounds, bonding fluids and inks. These materials are often dispensed from a syringe onto a preselected area. Heretofore this type of dispensing has been done manually for small jobs and repairs. Dispensing control units have been commercially available that pneumatically actuate a syringe under foot pedal control. This approach is too tedious and costly to be used on any significant size of production run.
Automated systems have been sold by ASYMTEK of Carlsbad, Calif. under the trademark AUTOMOVE (400 Series) for dispensing viscous materials. They have consisted of a benchtop X-Y positioner for carrying and guiding a variety of dispensing heads. The motions required may be programmed via an IBM-PC compatible computer. Menu-driven software is provided for programming the desired movements.
ASYMTEK has also sold another automated viscous material dispenser under the trademark DISPENSEMATE. See U.S. Pat. No. 4,967,933, granted Nov. 6, 1990, having named inventors Philip P. Maiorca et al. Predetermined pattern and fluid flow functions are selected for each of a plurality of consecutive movement elements by operator actuation of corresponding discrete manually actuable switches on a front panel without the need to program with a personal computer. This creates a workpiece program which can be automatically executed on command to dispense the viscous material over the upper surface of a workpiece such as a PC board in the prescribed manner.
In both the AUTOMOVE and DISPENSEMATE viscous material dispensers the height of the tip of the syringe relative to a fixed Z-axis scale may be pre-set prior to the dispensing operation.
In the manufacture of electronic circuit boards, laminated fiberglass PC boards with copper traces and pads are employed to electrically connect the leads of the associated electronic components. These leads are typically attached to the PC board in one of two ways. A first approach is to insert the component lead through a hole in a copper pad and subsequently solder the lead on the other side. This approach is known as the "through-hole" technique. A second approach is to solder the component lead directly to the copper pad without using a pre-drilled hole. This approach is known as the "surface mount" technique. This latter technique requires that a ball of solder paste be applied to the pad. The lead is then pressed into the paste. The components are held in place by the viscosity of the solder paste until the board is heated and the solder paste melts. When the solder solidifies, the component leads are firmly attached to the copper pads and have good electrical continuity therewith.
It is critical to proper attachment and conductivity via the surface mount technique that the solder paste be dispensed at a constant predetermined height above each pad. This ensures the generation of uniform tear-drop shaped balls of solder paste. Unfortunately PC boards are frequently warped. In order to compensate for board surface height variations expensive tools can be used to clamp the boards flat. It would be desirable, however, to automatically determine repeatable distances to the workpiece surface at different sites thereon to permit automatic height adjustment of the tip of the syringe prior to each dispensing of viscous material.
U.S. Pat. No. 4,661,368 of Rohde et al. granted Apr. 28, 1987 discloses a dispensing system for flowable materials such as adhesives which is designed to overcome the problems that arise because of height variations due to warpage in a circuit board. The dispensing nozzle tip is advanced into engagement with the particular position on the circuit board to which the material is to be applied. A reactive force on the nozzle tip from the circuit board is sensed by a load cell such that an exact spacing may be provided between the nozzle tip and the particular position on the board to which the material is to be dispensed.
U.S. Pat. No. 3,809,308 of Roeder et al. granted May 7, 1974 discloses a system for maintaining constant distance of a cutting torch from a work piece. A sensing electrode secured to the torch forms a capacitance with the surface portion of the workpiece, and a circuit senses changes in capacitance to adjust the height of the torch.
U.S. Pat. No. 4,762,578 of Burgin, Jr. et al. discloses another device for controlling the spacing between the tip of an adhesive dispenser and the surface of a circuit board. A laser diode and detector are mounted adjacent the tip. While the tip is being advanced, the image reflected off the circuit board moves along the length of the detector and provides an indication of the spacing between the tip and the circuit board.
The foregoing prior art approaches for detecting a predetermined syringe dispensing height have not been entirely satisfactory in commercial applications.